Indicative data from Russian and international literature sources on the extent of carbon sequestration by terrestrial ecosystems, mainly soils, at the global and regional levels are presented. It was noted, however, that these estimates were too approximate, highly debatable and require reliable experimental verification. It was suggested that close to real amounts of soil carbon sequestration in Russia and in the World will be obtained only in the future, with using data from long-term monitoring studies based on modern approaches and methods, including long-term field experiments. The terms of “soil carbon sequestration” were considered and a critical analysis of these definitions was given. Significant differences between the terms “soil carbon sequestration” and “soil carbon accrual” were noted, as well as the need to introduce into scientific discourse the term of “soil carbon depositing”, emphasizing the long-term preservation of carbon in the soil. It was pointed out that a complete quantitative assessment of soil carbon sequestration should include both the amount of organic matter input into soil and the gain in soil C_org, as well as the time during which carbon is retained in the soil. A list of the main reasons and factors limiting the process of carbon sequestration in soils was presented. The literature data on soil carbon accrual under different carbon sequestration agrobiotechnologies were summarized. It was concluded that the soils in the managed ecosystems, occupying significant areas in most countries of the world, have a significant potential for sequestration of atmospheric carbon and its transfer into soil organic matter. However, the technologies and approaches adopted for carbon sequestration does not guarantee a sustainable increase in C_org in the soil. Thus, the goal of climate- smart agriculture should be a reasonable compromise between climate and food aspects of the carbon problem by solving the triune aim of maintaining and/or improving soil fertility, increasing crop yields and mitigating anthropogenic carbon dioxide emissions.

The aim of the work was to briefly outline the main approaches to studying the structure of soil organic matter, allowing to obtain the most complete description of this unique natural phenomenon. The main attention is paid to the approaches implemented in the Dokuchaev Soil Science Institute, examples of which are published in this special issue. Present methods can be divided into two large groups: research and routine. Research methods are methods and approaches that have been recently introduced or are being introduced into scientific practice. Routine ones are widely used to study soil organic matter. The advantage of the former is the prospect of obtaining new unique data, while the latter provide the ability to obtain well-reproducible, comparable results that are promising for multivariate analysis. The research methods considered in the work include ion cyclotron resonance mass spectrometry with Fourier transform and nuclear magnetic resonance methods on 13C and 1H nuclei with Fourier transform. The most promising methods for analyzing the structure of soil organic matter are described as routine methods: optical methods (spectroscopy in the ultraviolet and visible range and fluorescence spectroscopy), infrared spectroscopy and pyrolysis with gas chromatography and mass detection.

The aim of the study was to investigate the effects of post- agricultural soil development on the respiratory activity and thermal stability of accumulated organic matter. A post-agricultural chronosequence formed on Haplic Luvisols and including currently arable land, 7- and 25-year-old post- agricultural abandoned land and grassland was studied. Soils were studied using thermogravimetric analysis aimed to investigate pools of thermally labile, stable and persistent soil organic matter (SOM). Elemental analysis was used to determine organic carbon and total nitrogen in bulk soil and water extracts. Basal and substrate-induced respiration rates were determined by incubating soils and measuring the amount of CO₂ released. The content and pool of soil microbial carbon were calculated based on the measured substrate- induced respiration. Organic, dissolved and microbial carbon, total and dissolved nitrogen and basal respiration rates increased during the post- agricultural development of soils. The specific respiration of microbial biomass was lowest in arable soil and increased after its abandonment. The availability of soil carbon for microbial decomposition increased in abandoned soils compared to arable ones. The SOM was dominated by the thermally labile pool (54–68%). The size of the thermally stabile pool of SOM was 19– 25%, persistent SOM was 13–21%. The accumulation rate of the thermally labile pool in the upper 30 cm of the soil, calculated for the 25-year period of the abandoned land use, was equal to 3.9 mg lab-SOM per year, stable –0.97 mg stab-SOM per year and persistent – 0.52 mg pers-SOM per year. As arable land was converted to abandoned, the proportion of the thermally labile pool in SOM increased. The post-agricultural increase in dissolved organic carbon was associated with the rise of the thermally labile pool of SOM, indicating the relationship between the availability of organic matter for microbial decomposition and its thermal stability.

Microorganisms play a key role in the dynamics of soil organic matter (SOM) stocks, redistributing carbon (C) among microbial biomass growth, respiration, polymer synthesis, and intracellular and extracellular enzymatic processes. This paper provides a review of microbiological indicators used to study the decomposition, transformation, and stabilization of SOM, as well as their application in modeling soil C dynamics. This study examines microbiological parameters of the carbon cycle, such as microbial biomass C, soil enzymatic activity, microbial necromass C, C use efficiency (CUE), basal respiration, and microbial community structure. Methods for determining these indicators, their interpretation, and examples of their application in mathematical models are discussed. Given that microbial necromass constitutes a significant portion of SOM and that CUE is a key parameter balancing C mineralization and stabilization, integrating microbiological data into predictive models can significantly improve their accuracy. Quantitative determination of microbiological indicators of the С cycle under various soil and ecological conditions is essential for studying the mechanisms of microbial transformation and stabilization of SOM.

This review article discusses the problem of developing and using methods for modeling soil organic matter (SOM) dynamics. Traditional methods based on “theoretical” discrete pools with different SOM turnover rates are critically analyzed, emphasizing their insufficient correspondence with actual observation data. An alternative approach considers the continuous distribution of SOM quality and allows us to understand and describe the mechanisms of transformation and stabilization of organic matter in soils under a wide range of soil formation factors and processes. Models of SOM dynamics based on this approach have greater predictive power for the development of agricultural practices aimed at increasing carbon sequestration in agricultural soils. This opens up new opportunities for preserving and improving soil fertility, as well as helping to respond effectively to global climate challenges in agricultural lands.

The article presents an analysis of the literature on labile and stable components of organic matter (OM) in agricultural soils. The labile components include light fractions (LF) identified by particle density (< 1.8 g·cm^-3), while the stable components include clay fractions (Clay) identified by particle size (<1–2 μm). Labile components of OM are very sensitive, while stable components are insensitive to changes in farming and land use systems. As a result, the ratio of carbon in the labile and stable pools, the C_LF/C_Clay ratio, is used as an indicator of the OM quality in agricultural landscapes. Physical soil fractionation methods used to isolate labile and stable components of OM are laborious and, therefore, not suitable for regional and global scale studies. The proposed theoretically substantiated express indicators of OM can be obtained using the proposed fairly simple granulometric fractionation method. These express indicators of OM will be characterized by different biogeochemical stability and their application for long-term and operational carbon monitoring in soils seems very promising. Experimental verification of theoretically justified simplified indicators is recommended in order to identify among them the correct indicators that most adequately reflect the impact of native and anthropogenic factors on the soil OM quality at different time scales.

Presently the problem of restoring the potential of cultivated lands in the Non-Chernozem Region of Russia is quite relevant. Thus, it is pointful to study the main factors influencing the fertility and stability of the soils in this zone. One of the leading factors altering the fertility, equilibrium, and stability of soils is organic matter. Its most active and labile in time and space component is dissolved organic matter. Dissolved organic matter actively interacts with living matter in soils and is interconnected with manifestations of biological activity. The aim of the work was to evaluate the optical properties of water-extractable organic matter (WEOM) and to reveal the relationship with the biological activity of sod-podzolic soils under different crops with different backgrounds of mineral nutrition elements. Absorption and fluorescence spectra were used to characterize the optical properties. Biological activity was evaluated by basal and substrate-induced respiration. As a result, it was shown that WEOM optical properties largely depend on the structure of the microbial community. At the same time, WEOM carbon content depended on the level of microbial activity, which, in turn, was largely determined by the presence of mineral nutrition elements. Fertilizer application stimulated microorganisms to process organic matter. At the same time, WEOM became more diverse and more humified.

The aim of this study was to assess the impact of barley (Hordeum vulgare L.) vegetation on the molecular composition of water-extractable organic matter (WEOM) in chernozem soils. The study employed a vegetation experiment method in a climate chamber, with soil samples taken before sowing and during barley vegetation. The molecular composition of WEOM was analyzed using gas chromatography-mass spectrometry (GC-MS). Based on the obtained data, the Shannon diversity index was calculated, and the contribution of different compounds to the composition of WEOM was evaluated. It was shown that barley vegetation increases the complexity of the WEOM composition in chernozem. The molecular composition of WEOM varied for all experimental conditions. The proportion of lipids and nitrogen- containing compounds in WEOM of chernozem decreased during barley vegetation compared to its pre-sowing state, which may be associated with their active microbial decomposition. There was a significant increase in the proportion of carbohydrates in the WEOM composition of chernozem during barley vegetation. The obtained data indicate the high sensitivity of the molecular composition of WEOM in chernozem soils to the influence of barley functioning and rhizospheric microorganisms.

Understanding the mechanisms underlying the accumulation and stabilization of organic carbon in soils is necessary to preserve and enhance their sequestration potential and to implement sustainable land use practices when converting soils to agricultural use. The aim of this work was to study the role of laccase in binding of phenolic acids to mineral phases and the role of laccase in organic carbon stabilization at low substrate concentrations occurring in soil solutions. The laccase of the white rot wood fungus Cerrena unicolor (VKM F-3196) was used as biotic catalyst. Laccase was immobilized on illite and on kaolinite modified with aluminum hydroxide – kaolinite-Al(OH)x. One of the common natural manganese (IV) oxides, pyrolusite (b- MnO₂), was taken as a powerful abiotic catalyst for comparison. The oxidative activity with 1 mM ABTS (diammonium salt of 2,2'-azino-bis-(3- ethylbenzthiozoline-6-sulfonic acid) as a substrate at pH 4.5 was 124 U/g for pyrolusite, 0.25 U/g for illite and was absent in modified kaolinite. The activities of laccase immobilized on modified kaolinite and illite were 1.17 and 0.82 U/g, respectively. An equimolar mixture of gallic, protocatechuic, p- hydroxybenzoic, syringic, vanillic and ferulic acids (0.01 mM each in 0.01 M KNO₃, pH 4.7) was incubated with minerals for 1, 24 and 72 hours. Phenolic acids loss was determined by reversed-phase high pressure liquid chromatography and carbon loss was determined on a TOC-L analyzer. The highest reactivity in interaction with all minerals was found for gallic acid (40–100% loss in 24 hours) and to a lesser extent for protocatechuic acid (19– 100% loss in 24 hours). Significant loss of p-hydroxybenzoic acid was observed only in the presence of illite and complex of illite with laccase, vanillic acid reacted only with pyrolusite (50% loss in 24 hours). The loss of syringic and ferulic acids (80–100% in 24 hours) was observed only in the presence of pyrolusite and complex of laccase with modified kaolinite. Despite 2 orders of magnitude lower oxidative activity and 3 times smaller surface area (18 m²/g versus 54 m²/g in b-MnO₂) the complex kaolinite- Al(OH)x-laccase adsorbed an amount of C_org comparable to pyrolusite (6.5 g/kg). The amount of carbon bound to complex of illite-laccase was 3 times lower (1.7 g/kg) despite the highest surface area of illite (100 m²/g) and catalytic activity, similar to kaolinite-Al(OH)x-laccase. Laccase enhanced carbon binding by modified kaolinite and illite by 2–3 times. Our results show the important role of laccase and metal hydroxides in C_org stabilization. Preservation and enhancement of the natural level of laccase activity in soils by regulating pH and humidity, as well as the introduction of laccase preparations in immobilized form into soils may be a promising approach to increase organic carbon stabilization potential of soils of agricultural use and requires further research in this area.

Stabilization of soil organic matter (SOM) is a key factor for maintaining fertility and reducing carbon dioxide emissions from the soil into the atmosphere during agricultural activities. A relevant scientific and practical area of research is the development of cultivation technologies that provide optimal physical properties of the soil for the plant growth and development, as well as for the vital activity of the soil microbiome. Understanding the physical mechanisms that regulate the carbon (C) balance of soils and the transformation of organic matter is therefore essential. This paper is aimed to provide an overview of existing concepts concerning the physical factors and mechanisms of C stabilization in soils, and to describe the physical processes regulating the C cycle in soils. The relationship between the SOM transformation processes and the physical factors of soil formation is shown through the modern understanding of the concept of the structural organization of soils, since SOM plays a key role in the formation of the soil structure and determines its quality. The development of methods and methodology of soil physics is analyzed and the most promising research areas for understanding the C cycle are considered. The review pays special attention to the influence of physical properties of soils on the growth and development of plants, as the main source of incoming organic matter and a necessary condition for sequestration of C by soils. The existing limitations in using of soil physical parameters in mathematical modeling of C stabilization processes are also considered.